Laminates which are produced by a co-extrusion process and which at least predominantly comprise thermoplastic material generally have at least over parts of their configuration at least three layers. Of those layers, at least one which serves as a support layer comprises thermoplastic material while at least one other layer of the laminate structure comprises a material which serves for producing a bonding action between the support layer and at least one further layer which in turn comprises a material having properties which differ from those of the material forming the support layer. Laminates of that nature, or end products which are produced therefrom, are employed predominantly for packaging purposes in which the properties of conventional plastic materials, more particularly for example polyolefins, do not meet all the requirements involved. A typical example is the permeability of polyolefins in relation to material to be packaged and/or the ambient medium around the package. Thus, the permeability of such materials may limit or hinder the use thereof as foils, liners or containers, for example as bottles, canisters, drugs, fuel tanks and the like. When using foils for packaging foodstuffs an important consideration for example may be that the packaging foils are impervious in relation to fats, aromatic substances or the like so that such substances which are part of the articles to be packaged in the foils do not diffuse through the foil to the exterior. If the foils were to be permeable to those and possibly other substances to be packaged therein, then the quality of the packaged material would alter in a disadvantageous fashion with the passage of time. For example in the case of bottles or like containers which are used for containing poisonous material or material which may have dangerous constituents, for example plant-protective agents, it is important for safety reasons to prevent the material contained in the bottles or like containers, or at least the dangerous constituents thereof, from diffusing through the material of the bottle or container, to the exterior. That consideration will apply in regard to a large number of chemicals which are packed in bottles and also canisters, drums or the like. When considering for example fuel tanks, a general requirement is that the walls thereof are not permeable in relation to hydrocarbons. In many cases however the problem may be that of preventing one or more substances from penetrating from the ambient atmosphere into the interior of packaging, a bottle, a container or the like, through the wall of the foil of the packaging or the wall of the bottle, container or the like. As a typical example in that respect, reference may be made to the requirement which is encountered in many situations of preventing oxygen in the air from diffusing through the foil of a package, the wall of a container or the like, into the interior thereof.
Although plastic materials are known which have such a low level of permeability for example in respect of gasoline and/or organic solvents that they do not exceed the permissible values, such plastic materials however tend to be expensive and/or cannot be readily processed. In many cases they also do not have the mechanical properties of for example polyolefins which make the latter suitable in respect of strength, toughness, elastic deformability and the like, precisely for use with materials whose transportation, storage and the like require certain precautions to be taken.
For the various reasons referred to above, foils, containers such as for example fuel tanks, and the like, are often produced in a multilayer or laminate construction, which involves at least one support layer of the laminate structure which generally comprises polyolefin being provided with a barrier layer which is more or less impermeable in relation to the material to be contained in the package or container, and/or the ambient medium. The barrier layer may for example comprise polyamide and the thickness thereof is generally substantially less than that of the one or more support layers. The barrier layer may be provided on the inward side and/or the outward side of the laminate structure. It is also possible however for the barrier layer to be arranged for example between two support layers, that is to say in a middle region of the cross-sectional configuration of the wall of the laminate structure. As however the different properties of the material forming the support layer on the one hand and the material forming the barrier layer on the other hand generally mean that it is not possible for the layers to be directly joined together in such a way that the joint therebetween is of adequate strength, an additional layer is often provided between the support layer and the barrier layer, the additional layer comprising another material which predominantly or exclusively is included to act as a bonding agent between the support layer and the barrier layer. The bonding agent layer must thus comprise a material which can form a bonding connection of adequate strength both to the material forming the support layer and also the material forming the barrier layer of the laminate structure. Although the bonding layer only has to perform an auxiliary function in the sense of producing a bond between the support layer and the barrier layer, nonetheless it is an aspect of crucial significance in regard to the mechanical strength of the end product made from the laminate, as if the bonding layer is incomplete, for example if it is missing in some parts of the laminate structure, that necessarily results in the mechanical strength of the end product being adversely affected as those parts of the laminate structure do not have a proper bond or only an incomplete bond, between the support layer and the barrier layer.
Both the barrier layer or layers and the bonding agent layer or layers are generally of small thickness. That is true in particular in respect of the layer or layers of bonding agent, the thickness of which may possibly be in the range of tenths of a millimeter, or even substantially less than that. Therefore, there is no guarantee under all the operating conditions which may arise in practical situations that those layers occur in the wall of the laminate foil or a hollow body or container produced therefrom, in a uniformly distributed configuration at all points where the presence thereof is required. As the endeavor, for reasons of economy, is to make the wall of the article produced from the laminate material and therewith also the individual layers forming the laminate structure, no thicker than is absolutely necessary, upper limits are set on the thickness of the individual layers making up the laminate. Accordingly, it is not reasonably possible totally to exclude the possibility that, on expansion or stretching of the foil or for example a preform for an article which is made from the laminate material, at least one of the layers of the laminate structure may be subjected to overstressing with the result that in the end product that layer is so incomplete or inadequate, in certain regions thereof, that it cannot perform the function for which it was intended.
Added to that is the fact that irregularities which are not visually perceptible may also occur in the process for forming the foil of laminate structure or a preform of laminate structure from which a hollow body such as a container or the like is to be produced, and possibly even when forming the individual layers forming the laminate structure, in an extrusion head. That may be the case in particular with foils and hollow bodies or preforms for producing same, which are produced in large quantities, for example on a mass-production scale. The production of such a foil or preform requires the use of a comparatively complicated piece of equipment comprising a plurality of extruders for plasticising the materials to be employed for the individual layers of the laminate structure, and a common extrusion head, into which the materials which have been plasticised in the individual extruders are passed in order to be brought together in the extrusion head and put into the required cross-sectional shape of the foil or the preform. In that situation also it is not possible completely to exclude the possibility that, even if only in exceptional cases, irregularities may occur in one layer or another of the laminate structure, which irregularities may possibly have the result, particularly in the case of layers which are in any case thin from the outset, that a layer is incomplete or is possibly even entirely missing, in a certain region of the foil or the preform produced in that fashion.
Generally speaking, defects or defective areas of that kind cannot be seen in the finished article, unless they occur in the outermost layer of the laminate structure thereof. However in normal circumstances that is fairly improbable as the outer layer or layers of a foil or hollow body is or are generally the above-mentioned load-carrying or support layer, or one thereof, which in any case is or are generally of greater thickness so that the occurrence of defects in such a layer is much less likely than in the case of the other layers, some of which are considerably thinner. Those layers which are thus at greater risk of being inadequate or defective are to be found predominantly in the interior of the wall structure of the laminate, that is to say between two other layers, so that they are not in any way accessibility from the outside. That applies in any event to the one or more layers of bonding agent as the only function thereof is to bond two other layers together so that a bonding agent layer necessarily always occurs between two other layers and is thus asked thereby on both sides. However, even the barrier layer which is intended to make the wall of the article impermeable is in many cases disposed between two other layers, generally two layers of bonding agent, so that it is also covered over thereby and is accordingly not accessible from the outside.
However, even if the barrier layer represents one of the outer layers of the wall structure of a hollow body of laminate nature, it is scarcely possible to perform a visual check in respect thereof as in such an arrangement the barrier layer generally forms the layer which defines the hollow interior of the hollow body and it is therefore not readily accessible. Added to that is the fact that an exclusively visual check is in any case extremely difficult to carry out, without additional aids for making any defects or defective areas visible. At any event, even if the layer to be checked were on the outside of an article formed by a laminate structure, visual checking would have to be carried out with an extremely high level of care in order satisfactorily to detect defects or defective areas. That applies even when, when considering a hollow body such as a container in which the barrier layer to be investigated is disposed on the inward side of the laminate structure thereof, as referred to above, samples of the hollow bodies were to be cut open in order to carry out sample testing thereof.
It has already been mentioned above that a satisfactory set of characteristics in respect of the laminate in the form of a foil or a container may be a matter of crucial significance, particularly when dealing with sensitive and/or dangerous materials to be packaged therein, in regard to the aspect of mechanical strength of the foil or container, not least also from the point of view of product liability. Undoubtedly it is a matter of disadvantage that multi-layer foils and containers or the like which are produced by a co-extrusion process can at present be checked visually or in some other fashion, only to a very incomplete and inadequate extent, in regard to their satisfactorily complying with the requirements made in respect thereof.